In recent years, there has been proposed a magnetoresistive random access memory (MRAM) which makes use of a tunneling magnetoresistive effect. The MRAM has a feature in that data is stored by a magnetization configuration of an MTJ (Magnetic Tunnel Junction) element. As a method of changing the magnetization configuration, there is known a method of using a magnetic field which is generated by an electric current flowing in wiring, or a method of using a spin-polarized current. In the former method, since it is necessary to dispose the wiring, which generates the magnetic field, near the MTJ element, there is a problem that integration is difficult. On the other hand, the latter method has a feature that integration is easy, since a memory cell is basically composed of a select transistor and an MTJ element, like a DRAM.
The MTJ element includes a reference layer having a fixed direction of magnetization, a memory layer in which the direction of magnetization is changed by a spin-polarized current, and a tunnel barrier layer disposed between the reference layer and the memory layer. The value of an electric current, which passes through the tunnel barrier layer, varies depending on whether the direction of magnetization of the memory layer and that of the reference layer are identical or opposite. Based on this variation in resistance, data 1 and data 0 are discriminated. Rewrite of data is executed by causing a current to flow through the tunnel barrier layer, thereby transferring spin polarization of the reference layer to the memory layer. As the reversion of magnetization of the memory layer is easier (i.e. as the magnetic anisotropy is smaller), the direction of magnetization of the memory layer can be changed by a smaller current. Accordingly, a current, which flows through a select transistor that is connected in series to the MTJ element, can be decreased, and microfabrication of elements can advantageously be achieved. Meanwhile, if the magnetic anisotropy of the memory layer is decreased, such a problem arises that the direction of magnetization is reversed by thermal agitation and the data retention characteristic deteriorates.